1. Technical Field
The present disclosure relates generally to the field of computational algorithm used in lithography and optical proximity correction (OPC).
2. Background Art
To ensure that specific features of very large scale integrated circuits can be printed, mask shapes most often require manipulation to ensure manufacturability. Very often, this means that the original layout representing the designers' intent bears little or no relationship to mask shapes created to ensure target dimensions are achieved on wafer. These shapes are chosen during OPC to maximize pattern transfer to the wafer and a process window. Square contacts in layout design have a rich history in the semiconductor industry. In fact, they have been used almost exclusively to simplify the manufacturing process of the mask as well as to transfer contacts to a wafer. Due to increased technology demands and ever shrinking ground rules, the use of single sized contact structures has been relaxed to include several different geometries for contacts. In particular, current technology critical dimensions are challenging the resolution of current lithographic tooling resulting in severe pattern dependent instabilities and severely reduced process windows for single size contact configurations. As such, mask critical dimension (CD) tolerance specifications for contact levels that use square contacts need to be extremely tight (compared to line levels) to accommodate high two-dimensional mask error enhancement factor (MEEF). Even if single point contrast and depth of focus (DOF) were acceptable, much higher MEEF in combination with typical mask manufacturing CD errors are limiting across chip line variation (ACLV) and would require non-competitive mask specifications.
Since MEEF is to the first order driven by mask dimensions, mask error factors are compounded for square contacts since they are inherently two-dimensional features. There exist several options to overcome these difficulties—none of which provide complete solutions for shrinking contact configurations: 1) Avoid square or effectively square contact targets of minimum dimensions at minimum pitches. This approach is often not feasible due to circuit related contact requirements (see German patent DE10126130A1). 2) Tighten mask CD tolerance specifications. This approach is often not possible since mask making tool sets are typically operated at their limits. Additionally, mask costs will increase if tolerance specifications are tightened because more mask manufacturing resources will be required to create masks that meet more aggressive tolerance specifications. 3) Change illumination conditions. In a real world manufacturing environment, there is typically not much to gain with this approach without significantly sacrificing overall process window.
Overall, in a world with optical proximity effects, non-negligible corner rounding, and 1-dimensional MEEF values already significantly larger than 1.0, the advantages of using single contact geometries are significantly diminished by increased manufacturing costs and decreased process windows.